Silicon-based materials, where silicon is the primary material of construction, are employed in numerous integrated circuits (IC) and microelectromechanical systems (MEMS) devices. However, it has long been known that in aqueous chemical environments, where silicon-based sensors and actuators may be used, that corrosion (etching) of the silicon-based materials can cause premature device wear and failure. In fact, there are many commonly used processes for machining silicon that rely on wet corrosion (etching) of silicon and silicon-based materials such as silicon oxides and silicon nitrides; see for example Kendall, D. L.; Shoultz, R. A. “Wet Chemical Etching of Silicon and SiO2, and Ten Challenges for Micromachiners”, SPIE Handbook of Microfabrication, Micromachining, and Microlithography, Vol. 2, SPIE Optical Press, pp. 41-97, 1997. Ed. P. Rai-Choudhury. Recently, MEMS technology has been applied to fluid management systems employed in inkjet printing systems.
Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand (DOD) inkjet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal inkjet (TIJ)) and piezoelectric transducers. In another process known as continuous inkjet (CIJ), a continuous stream of droplets is generated and selected droplets are separated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught and recycled to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Most types of inkjet printers employ a printing head made from silicon-based materials including silicon, silicon dioxide, and silicon nitride because these materials are common in semiconductor fabrication facilities and can be readily processed to form highly complex integrated circuits and electromechanical devices. Parts of the print head including the printing nozzles as well as the channels that feed ink to the print head and printing nozzles often contain regions of these silicon-based materials that are in direct contact with the printing ink. It is well known in the art that a wide range of ink compositions can cause these silicon-based materials to dissolve or induce stress that results in mechanical failure and increased rates of dissolution (U.S. Pat. No. 6,730,149 B2). The reliability of the inkjet printing device can be dramatically reduced by these interactions between the ink and the silicon-based materials in the print head.
Continuous inkjet (CIJ) printers typically consist of two main components; a fluid system and a print head, or multiple print heads. Printing fluid such as ink is pumped through a supply line from a supply reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the print head. Stimulations are applied to the print head to cause those ink streams to form streams of spaced droplets, which are deflected into printing or non-printing paths. The non-printing droplets are returned to the supply reservoir via a droplet catcher and a return line. U.S. Pat. Nos. 3,761,953 A, 4,734,711 and 5,394,177 and EP 1,013,450 describe in detail the design of a fluid system for CIJ apparatus. The more recent development of a silicon-based MEMS CIJ printhead fabrication and printing apparatus can be found in U.S. Pat. No. 6,588,888 and U.S. Pat. No. 6,943,037, the disclosures of which are herein incorporated by reference. The design of the nozzle plate (printhead die) used in the drop generator of the printing system is one of the distinguishing elements of MEMS CIJ technology. A single crystal silicon die may be used as the substrate for the nozzle plate and, complementary metal oxide semiconductor (CMOS) electronics are included as part of the device. The surface nozzle structures and associated on-board CMOS electronics are fabricated using the same manufacturing technologies and material sets employed for the construction of silicon integrated circuits. The printhead die also incorporates fluid channels running through the silicon. During drop generation, heaters in the device transfer thermal energy to the fluid jetting through each nozzle.
As noted in the discussion above, the CIJ printhead is comprised of several components. A more detailed discussion of the printhead and its operation is provided herein with particular emphasis on silicon and its interactions with fluids, given that silicon-fluid interactions are particularly relevant to the present invention. These components include a manifold for interfacing with the fluid system and accepting ink or other fluids supplied by the fluid system to allow transport of these fluids to other components of the printhead; an electrical interconnect system means for interfacing with the electrical signals supplied by an external writing system that supplies the printhead with the information pertaining to the drop-wise formation of a printed image on a support, where the support is stationary or non-stationary, from ink-containing drops generated by the printhead; and a drop-generating component, whose function is to provide a means for generating drops from ink or other fluids delivered to the drop generating component from the manifold. The drop-generating component providing a means for generating drops in a silicon-based CIJ printing system employs silicon-based devices fabricated using the same technology employed for fabricating silicon integrated circuits. The silicon-based devices may contain multiple fluid channels as well as a plurality of small orifices, also called nozzles, which enable ink or other fluids supplied by the fluid system to pass from the manifold to the support through the formation of one or more columns of fluid also called fluid jets, which exit the silicon-based device when appropriate pressures are employed. The fluid column(s) or fluid jet(s) transform into well-defined drops under appropriate conditions. The pressures employed in silicon-based CIJ printing system are generally above 69 kPa and less than 1380 kPa. The materials of construction of the silicon-based devices in a silicon-based MEMS CIJ printhead may be quite varied and the materials of construction that contact ink or other fluids supplied by the fluid system or manifold are of particular interest to the present invention.
Silicon-based devices used as components that provide a means for generating drops from a fluid are generally fabricated using substrates prepared from single crystal silicon. The use of large grain polycrystalline silicon substrates for device fabrication is known in the art. The substrates may have varying thicknesses, from 50 microns to greater than 1 mm, and the substrate surface may have any crystallographic orientation that is suitable for the device application. For example, the silicon substrate may be prepared with an orientation defined by Miller indices of <100>, <111>, <110>. The use of various crystallographic orientations in device substrates is well known to those familiar with the art of semiconductor device fabrication. The crystal orientation of single crystal silicon is commonly indicated thusly; Si(100), Si(111), or Si(110) for example. The single crystal silicon substrate may have varying electrical properties. For example, the electrical properties of the single crystal silicon can be varied by the incorporation of small amounts of foreign impurities, also called dopants or carriers. These foreign impurities, such as, for example, boron or phosphorus, determine whether the electrical charge of the majority carrier type in the silicon crystal is negative or positive. Such modified substrates are known as n-type and p-type silicon, respectively. The use of both p and n-type silicon substrates for fabrication of silicon-based devices is known in the art. The use of silicon substrates of low resistivity, where the resistivity is less than 100 ohm-cm, and the use of silicon substrate of high resistivity where the resistivity is greater than 1000 ohm-cm, irrespective of carrier type and substrate crystallographic orientation, is known in the art of semiconductor device fabrication.
The additional preparation of substrates by deposition of layers of silicon, either polycrystalline or amorphous by various means as well as deposition of silicon by various means on insulating layers prepared by various means, such as, for example, polysilicon deposited on silicon dioxide insulators formed by thermal oxidation of the silicon substrate, also known as silicon on insulator or 501, is known in the art. The resulting deposited silicon containing layer(s) may be either doped or undoped, p-type or n-type, and additionally may be either polycrystalline, meaning that the arrangement of silicon atoms in three dimensional space within the layer are identical with those found in single crystal silicon, or amorphous or poorly crystalline, meaning that that the arrangement of silicon atoms in three dimensional space within the layer deviates relative to those found in single crystal silicon and shows varying degrees of disorder relative to those atomic positions found in single crystal silicon. Device performance has been shown to improve after substrate surface quality has been controlled by the use of additional layer deposition, and this observation is familiar to those knowledgeable in the art of semiconductor device fabrication.
The use of subsequently deposited layers optionally containing silicon is known in the art of semiconductor device fabrication. Deposited layers optionally containing silicon can be prepared by any method known in the art of semiconductor device fabrication including chemical vapor deposition with the optional use of plasma assistance or enhancement at low (<400° C.) and high temperatures (>400° C.) under both low pressure (<1 torr) and high pressure (>1 torr) conditions. Deposited layers optionally containing silicon can be prepared by vapor deposition by physical vapor deposition (evaporation) optionally plasma assisted or enhanced, as well as by epitaxial growth methods. The resulting optionally silicon containing layers may be electrically insulating or electrically conductive to varying degrees, either doped or undoped, p-type or n-type, and additionally may be either polycrystalline, meaning that the arrangement of atoms in three dimensional space within the layer are identical with those found in single crystals of the same elemental composition, or amorphous or poorly crystalline, meaning that that the arrangement of atoms in three dimensional space within the layer deviates relative to those found in single crystal of the same composition and shows varying degrees of disorder relative to those atomic positions found in single crystal silicon. It is known in the art that silicon containing deposited layers may contain additional foreign atoms of varying amounts including, for example, some of the aforementioned dopants boron and phosphorus to control electrical properties, and additional atoms, interstitial or otherwise, resulting from the deposition process or a combination thereof. Examples of dopants include boron, phosphorus, arsenic, nitrogen, carbon, germanium, aluminum, and gallium. Examples of interstitial or non-interstitial foreign atoms include hydrogen, oxygen, nitrogen, carbon, select atoms from elements listed from group VI B of the periodic table (O, S, Se, Te) and select atoms of elements listed in the group VII B of the periodic table (F, Cl, Br, I). Hydrogen, oxygen, nitrogen, and carbon are commonly present with silicon in devices and devices containing microelectromechanical systems and each of the elements oxygen, nitrogen and carbon are often found combined with silicon in the form of stoichiometric or non-stoichiometric binary, ternary, and quaternary compounds like silicon hydrides of varying compositions, silicon oxides of varying compositions and hydration including silicon suboxides and hydrated silicon oxides and suboxides, silicon nitrides of varying compositions, silicon oxynitrides of varying compositions, silicon carbides of varying compositions, and silicon oxycarbides of varying compositions. In addition, a variety of glass compositions are commonly employed in microfabrication, and they have different mechanical and chemical properties. Undoped silicate glass layers (USG) can be made from several starting materials and processes. Spin-on-glasses (SOG) from tetraethylorthosilicate (TEOS) decomposition, which provide excellent uniformity and step coverage, are attractive in microprocessing. Doping with phosphine produces a phosphosilicate glass (PSG), and additionally including boron produces a borophosphosilicate glass (BPSG). Doping increases the wet and dry process etching rates, and the softening temperatures for processing flexibility during device fabrication. These binary and ternary silicon containing compounds can be either discrete layers in the device or part of the surface composition of silicon, polysilicon, and amorphous silicon. Additionally, other elements such as Al, Ti, Ta, W, Zr, Hf, and Cu are often found with silicon and/or silicon containing binary compounds such as silicon oxides and silicon carbides, in devices and are sometimes observed as intermetallic alloys with silicon. Examples of intermetallic silicon containing alloys are titanium containing silicides of all compositions, tantalum containing silicides of all compositions, tungsten containing silicides of all compositions, zirconium containing silicides of all compositions, halfnium containing silicides of all compositions, copper containing silicides of all compositions, as well as ternary aluminum silicon oxides, ternary halfnium silicon oxides, ternary zirconium silicon oxides. Those knowledgeable in the art of semiconductor device fabrication are familiar with the different alloys, binary compounds, ternary and quaternary compounds that can form during processing and this is considered common knowledge in the art.
When a continuous inkjet printing system is in operation, fluid is essentially always flowing through the nozzles of the drop generator. There may be startup fluids passing through the printer for cleaning the fluid delivery system before printing with inks. Inks may remain in the printing system for extended times during a given printing run because the run duration may vary from hours to weeks. Flushing fluids may be used during ink changeovers or as part of routine maintenance. When the system is printing, only a small portion of the ink passing through the drop generator actually prints on the substrate. Most of the ink is collected and returned to the fluid delivery system for reuse. Finally, shut down fluids and storage fluids may be used to clean out inks from the fluid delivery system and the printhead, and ensure that the system does not fail during startup after storage.
It is desirable to have a print head operate reliably for many hundreds to thousands of hours. The fluid volume passing through a CIJ print head is large; accordingly, over a desired print head lifetime, many thousands of liters of solution can pass through the print head die. Therefore there is extensive exposure of the silicon-based nozzleplate to fluids in CIJ systems. Any degradation of the silicon-based materials in these solutions, as by corrosion (or etching, or dissolution), represents a great concern.
While typically not exposed to the same volume of fluid as are CIJ print heads, drop-on-demand inkjet printing systems employing printing heads made from silicon-based materials can similarly be impacted by undesired degradation of the silicon-based materials upon exposure to aqueous inkjet printing fluids. This problem has been difficult to solve. In some inkjet printing systems, the silicon-containing portions of the print head, in particular the ink chamber, the nozzles, and the ink channels are replaced along with the ink cartridge so that the lifetime of the silicon-based materials is limited to the lifetime of the individual ink cartridge. This approach dramatically increases the cost of the ink cartridge, and limits the printing system design.
Another approach to preventing aqueous printing fluids from dissolving the silicon-based materials has been to coat or deposit a resistant material on all the surfaces that come in contact with the printing fluid. These coatings can be either organic such as polymers or inorganic such as oxides of titanium or hafnium. This method also has the drawback of increasing the cost of the print head and often is also plagued by poor uniformity or pinholes in the coating that limit the protection from the printing fluid.
There is a strong need for ink compositions that do not dissolve or damage the silicon-based materials in the print head.
Ink compositions containing colorants used in inkjet printers can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. Pigments are highly desirable since they are far more resistant to fading than dyes. However, pigment-based inks have a number of drawbacks. Great lengths must be undertaken to reduce a pigment to a sufficiently small particle size and to provide sufficient colloidal stability to the particles. Pigment-based inks often require a lengthy milling operation to produce particles in the sub-micron range needed for most modern ink applications. If the pigment particles are too large light scattering can have a detrimental effect on optical density and gloss in the printed image.
A second drawback of pigmented inks is their durability after printing, especially under conditions where abrasive forces have been applied to the printed image. Furthermore, the images printed onto an inkjet receiver are susceptible to defects at short time intervals, from immediately after printing to several minutes while the inks are drying. Finally, the durability of the dried image is also subject to environmental factors such as temperature and humidity which, under certain circumstances, can degrade image durability. To this extent, pigmented inks have been formulated with various polymers, dispersants and other addenda in attempts to provide durable images that can withstand post printing physical abuse and environmental conditions.
A number of approaches to reducing the propensity of the ink to dissolve or damage the silicon-based print head materials have been disclosed. Dissolution of silicon-based, drop-on-demand thermal inkjet (TIJ) printhead components, especially components comprised of silicon oxides or silicates to form soluble silicates has been reported to be inhibited by the addition of suitable trivalent metal ions such as Al (III) or Fe (III) as disclosed in WO 2009/035944A3 to Yue at al, the disclosure of which is herein incorporated by reference. Because the metal salts can inhibit dissolution of silicon oxides, they can also be effective in the inhibition of silicon metal etching by preventing dissolution of the native silicon oxide present on silicon metal. Disclosed metal salts include aluminum nitrate nonahydrate and ferric nitrate, added in amounts to provide about 10-50 ppm of metal ion. U.S. Pat. Nos. 6,435,659 and 6,607,268 B2 to Bruinsma and Lassar similarly disclose use of aluminum salts added to drop-on-demand thermal inkjet ink to form a passivating protective layer on the resistive heater of the printhead. While addition of aluminum organic chelate complexes and aluminum metal are noted as alternative sources to aluminum salts for the aluminum added to the ink, all examples employ aluminum nitrate, and certain examples actually disclose that organic acids added as buffers can act to chelate the aluminum ions and prevent formation of the desired passivating film. In a similar vein, Arita et al. in U.S. Pat. No. 6,730,149 B2 disclose the stabilization of silicon-based piezoelectric print head devices against the debilitating effects of device corrosion by the inclusion of metal cations such as zinc (II) and aluminum (III).
There are, however, unsatisfactory effects associated with the inclusion of multivalent salts in inkjet inks, especially the preferred aluminum ion. Widely used, soluble azo dye colorants (e.g., Direct Black 19, Direct Red 28, Direct Blue 86, etc.) can react with solvated aluminum ion at normal, alkaline ink pH levels, and produce insoluble matter—solvated aluminum ion has been investigated as a waste water purification coagulant treatment to remove such dyes specifically, as supplementing its widespread use for general soluble organic material. Pigmented inks frequently contain organic polymer binders or dispersants as noted above, which are typically solubilized at weakly alkaline pH levels by deprotonation of carboxylic acid groups. The resultant carboxylate ions form complexes with multivalent metal ions, and the polymers may further crosslink and become insoluble. Paper substrates for inkjet printing are often treated with calcium ion or other multivalent salts, for example, to precipitate the colorants and binders at the paper surface, resulting in increased reflection optical density. Furthermore, a very complicated, pH-dependent chemistry results from the simple dissolution of an inorganic aluminum salt in water. A pH-neutralized aluminum ion solution initially may contain mononuclear aqua complexes or multinuclear polyoxocation cluster, but these species are all unstable to eventual polymerization and formation of sols, or dispersed colloidal aluminum hydroxide or hydroxyoxide material that will eventually precipitate. Thus dilute aluminum reagent solutions are needed to avoid severe reactions with typical inkjet ink components during ink mixing. The useful lifetime of the aluminum reagent solution used to make an inkjet ink is reduced, and the inkjet ink quality and shelf-life is also affected. The formation of any precipitates causes numerous problems: the multivalent metal passivating agent is removed from the ink fluid enabling the resumption of printhead corrosion; precipitates can lead to filter pore blocking or jetting orifice blocking; deposit build up on printhead components such as the nozzle plate can produce jets that are misaligned and result in printing errors; contamination of TIJ heater surfaces can cause misfiring or complete drop ejection failure.
Another approach employs dispersions of specific metal oxide particles such as alumina or cerium oxide where the particles have a positive charge in the pH range from 4 to 6 (US2008/012981 A1) as characterized by their positive zeta potential in this pH range. These positively charged particles are believed to adhere to the negatively charge silicon-based surfaces in the print head and thereby eliminate the dissolution of these surfaces by the ink. These methods are limited to specific ink compositions and may not work well with pigment-based inks where a negative charge on the pigment surface and polymers is critical to the stability of the pigment dispersion and resulting ink. Another general approach to improving ink performance with regard to silicon corrosion is through adjustment of the ink pH value through the use of appropriate buffer solutions. For example, Inoue et al. in U.S. Pat. No. 7,370,952 B2 note that buffers can be used to adjust the pH values of inks used in drop-on-demand inkjet printers to reduce the effects of corrosion. This is primarily because the corrosion of silicon is known to be accelerated by higher pH value (more alkaline) solutions, such as those used in wet etching processes. At the same time, compositions useful to inkjet inks often require some alkalinity in order to maintain solution integrity, e.g., in order to prevent precipitation of ink components.
An improvement in silicon, silicon oxide and glass passivation reagents for inkjet printing fluids is needed.